SENSORY SYSTEMS
General Overview
Sight, sound,
touch , pain, smell, taste and the sensation of bodily movement
originate in the sensory system.
These perceptions
form the basis of our knowledge of the world.
Sensory systems
receive information from the environment through receptors at the
periphery of the body and transmit this information to the
central nervous system .
Sensory
information is used for three main functions à sensation,
control of movement and maintaining arousal.
Not all sensory
information is perceived e.g. sensory information used to control
movement.
We also receive
sensory information from within the body e.g. blood vessels and
viscera.
This information
is used to regulate temperature, blood pressure, heart rate,
respiratory rate and reflex movements.
Regulation of
these bodily functions usually does not reach conciousness
Sensory information travels to the brain via specialized pathways for each type of receptor (labeled- line code of stimulus quality)
General Properties of Sensory Systems
Fig 8-2
All sensory
pathways require
(i)
a stimulus e.g. light, pressure, sound
(ii)
a sensory receptor à transduces the stimulus into a graded
electrical signal i.e the amplitude of the graded potential is
related to the intensity of the stimulus.
Each type of sensory receptor has an
adequate stimulus that is unique to that receptor e.g.
photoreceptors in the eye are sensitive to light but not to
sound.
The adequate stimulus will produce a
change in the membrane potential of the sensory receptor to
generate a generator or receptor potential.
The generator potential can be either a
depolarization e.g. somatic sensory receptors or a
hyperpolarization e.g. photoreceptors in the eye.
The generator potential, if it produces a sufficient level of depolarization at the trigger zone of the axon will fire an action potential or a series of action
Sensory Modalities
Different forms of energy are transformed
by the nervous system into different sensations or sensory
modalities
Five major sensory modalities are
recognized:
(i)
vision
(ii)
hearing
(iii)
touch
(iv)
taste
(v)
smell
Each modality has many constitutive
qualities or submodalities e.g. taste can be sweet or sour.
Sensory Receptors
Sensory receptors are divided into 5 major
types based on the type of stimulus to which they are most
sensitive
(i)
Chemoreceptors à respond to chemical ligands that bind to
the receptor
(ii)
Mechanoreceptors à respond to various forms of mechanical
energy such as pressure, vibration, sound.
(iii)
Thermoreceptors à respond to temperature
(iv)
Photoreceptors à respond to light
(v)
Nociceptors à respond to noxious (painful) stimuli
.
Somatosensory
receptors are receptors that respond to external stimuli that
affect the skin or the surface of the body.
The major types
of somatosensory receptors are
(i)
tactile receptors activated by touch, pressure and vibration
(ii)
thermal receptors: two types
cold receptors à respond to temps below 30oC
warm receptors à respond above 30oC
(iii)
nociceptors à two types
mechanical
à respond to intense mechanical stimulation
heat nociceptors
à respond to temps above 45oC
(iv)
proprioceptive receptors à Respond to position and
movement of limbs.
Receptors vary
widely in their levels of complexity
Somatosensory
receptors are the simplest consisting either of naked nerve
endings or nerve endings encased in connective tissue capsules
e.g. Pacinian corpuscle..
The special
senses have sense organs that consist of specialized receptor
cells that synapse onto sensory cells à e.g. hair cells
found in the ear. The hair cell synapses onto a sensory neuron.
Activation of the receptor cell trigers release of transmitter
which depolarizes the sensory neuron and initiates an action
potential.
Transduction Mechanisms
All transduction mechanism involve
(i)
an adequate stimulus
(ii)
a generator potential
(iii) the initiation of action potentials
Sequence of events for sensory
transduction in Pacinian Corpuscle (Fig 8-3)
Pacinian
corpuscle à a somatosensory receptor located under the
skin that responds to touch.
Consists of free
nerve endings encapsulated by layers of connective tissue
Pressure is the
adequate stimulus.
Pressure causes
compression of connective tissue and consequently the free nerve
endings
Produces opening
of Na channels embedded in the membrane
Influx of Na ions
depolarizes membrane to produce a generator potential
If generator
potential is of sufficient amplitude to depolarize the initial
segment of the axon to threshold to initiate an action potential,
an action potential will be initiated.
Frequency and Population Coding (Fig
8-4; see also Fig 1 in syllabus notes)
A method of informing the brain about the intensity
of a stimulus.
(i)
Frequency code
The greater the
stimulus intensity e.g. pressure for a Pacinian corpuscle, the
greater the number of action potentials initiated in the sensory
axon associated with that receptor.
(ii)
Population code
A greater stimulus intensity (pressure) produces more action potentials by activating more receptors
Labeled-line Code of Stimulus Quality
How the nervous
system codes for the type (quality) of stimulus detected by the
sensory receptor.
The information
about the temperature of the skin travels to the brain via a
different pathway than does information of, for example,
pressure.
Different types of receptor have different sets of connections in the CNS
Sensory Adaptation (Fig 8-6; see also
Fig 2 in syllabus notes)
If a stimulus is
applied for an extended period of time at some point the brain
ceases to perceive it e.g. background noise such as a ticking
clock.
Adaption
mechanisms can occur:
(i)
at the receptor - e.g. in Pacinian corpuscle the free nerve
ending returns to its original resting shape even though the
pressure is still maintained. Na channels close, no generator
potential
(ii)
at the molecular level à e.g. Ãbleachingà of
photoreceptors in response to the brightness of a sunny day.
Receptors that capture light are removed.
Rapidly and Slowly Adapting Receptors
(i)
Rapidly Adapting (Phasic
receptors)
Receptors
quickly adapt i.e. stop generating generator potentials in
response to the stimulus e.g. Pacinian corpuscles. Also hair
receptors in hair follicle on surface of the skin. Generally,
once adaptation has occurred the only way to reactivate receptor
is to change the intensity of the stimulus.
.
(ii) Slowly
Adapting (Tonic receptors)
Receptors
continue to fire action potentials as long as stimulus is applied
e.g. Ruffini ending receptors found in the skin that are
sensitive to stretch of the skin.
Each sensory modality is asociated with a particular nerve fibre of a certain diameter
(i) Large diameter (13-22um) myelinated fibers transmit info about touch and pressure. A beta fibers. They have fast action potential conduction velocities (70-120m/sec)
(ii) Small diameter (1-5um) myelinated and unmyelinated fibers transmit info about temperature and pain. A delta and c fibers. They have slow conduction velocities (2-15m/sec)
The different classes of sensory (primary afferent) fibers that convey somatosensory modalities take specific routs and end in different regions of the spinal cord.
Runs from base of head to the small of the back.
Terms used to describe location of nuclei and fibre tracts in the spinal cord.
Longitudinal orientation
(i) rostral - toward the nose
(ii) caudal à toward the tail
(i) dorsal à towards the back
(ii)
ventral à towards the back
Also
(i) medial à close to midline
(ii) lateral à to the side of the body
(iii) anterior à towards the front
(iv) posterior- towards the back
Spinal cord divided into 4 regions corresponding to the adjacent vertebrae:
(i) cervical
(ii) thoracic
(iii) lumbar
(iv) sacral
In cross section the spinal cord has as a butterfly ot H shaped core of gray matter (cell bodies) and a surrounding rim of white matter (axons).
White matter can be divided into a number of columns composed of tracts of axons that transfer info up and down the cord.
There are two major pathways that carry sensory information:
(i) dorsal column pathway à carry info about touch and pressure. Nerve fibers do not cross over the midline in the spinal cord.
(ii) anterolateral pathway à carry info about pain and temperature. Nerve fibers cross over midline at level of spinal cord.
Spinal cord consists of 31 segments:
(i) C1-C8 Ã sensory info form back of head, neck, shoulders, part of the arms and hands
(ii) T1-T12 Ã parts of the arms and hands and trunk of the body
(iii) L1-L5 Ã waist, thighs,upper and lower legs, parts of the feet
(iv) S1-S5 Ã back of legs, buttocks, anus
Each segment receives information from particular areas of skin. These areas are called dermatomes. Important diagnostic tool for locating a site of injury to the spinal cord and dorsal roots.
Sensory fibers enter spinal cord through the dorsal root and ascend toward the brain in the dorsal portion of the white matter.
Synapse with second order sensory neurons in the dorsal column nuclei.
Fibers cross over the midline (tract decussates) and become part of medial lemniscus pathway.
Fibers project to and synapse with cells in the thalamus.
Fibers project to and synapse with cells in the cortex.
The thalamus often described Ãrelay stationÃ
Receives visual, auditory, and taste info in addition to somatic info.
Somatic sensory input processed in the ventral posterior lateral (VPL) nucleus.
Neurons in VPL transmit info to the neocortex.
The neocortex is divided into discrete regions that receive somatic, visual, auditory, guststory sensations.
There are also regions devoted to the control of movement and other functions e.g. speech
The somatic sensory cortex is topographically organized i.e. sensory information from the various body areas are concentrated in discrete areas of the cortex e.g. sensory info from the legs is sent to the medial portion while info from the arms, head and face is sent to the most lateral regions of the cortex.
The amount of space of the sensory cortex devoted to each part of the body is proportional to the sensitivity of the body part. Thus, areas in the cortex representing the thumb and fingers of the hand occupy substantially more space than that represented by the torso.
Somatic sensory cortex also organized so that all of the cells that respond to one type of sensation are located together within vertical columns. ß
The area of skin that a sensory neuron innervates is the receptive field for that particular neuron.
Stimulation with the appropriate stimulus within the receptive field will activate the sensory neuron associated with that receptive field.
Receptive fields of individual sensory neurons frequently overlap with the receptive field of neighboring receptors
Primary sensory and secondary sensory neurons do not always exist in a 1:1 ratio i.e. multiple primary sensory neurons converge onto a single secondary sensory neuron. Thus the receptive fields of secondary sensory neurons are often larger than primary sensory neurons.
This allows simultaneous subthreshold stiumuli to summate at the secondary sensory neuron and initiate an action potential.
The size of the secondary sensory neuronÃs receptive field determines the sensitivity of an area to a stimulus.
Sensitivity can be demonstrated with the two point disrimination test.
Very sensitive areas e.g. finger tips. There is not much convergence of sensory input at the second order level. Receptive fields of secondary sensory neurons are small. Thus two stimuli separated by only a few mm will be perceived as distinct stimuli.
Less sensitive areas e.g. arms and legs. There is a lot of convergence of sensory input at the second order level. Receptive fields of secondary sensory neurons are large. Thus two stimuli applied close together are perceived as only one stimulus.
A way the location of a stimulus can be isolated.
Example:
Activation within a receptive field at point B activates three sensory neurons. Ã B and neurons A and C whose receptive fields are adjacent to B.
Sensory neurons A B and C all have a tonic (baseline) level of activity that increases following activation with the stimulus.
Neurons A B and C synapse with secondary sensory neurons in the dorsal column nuclei. Sensory neurons in the dorsal column nuclei also generate a low rate of spontaneous action potentials i.e. baseline activity that increases in response to the stimulus.
Secondary sensory neuron B when activated increases its baseline level of activity.
Neuron B also has inhibitory input to neurons A and C. Thus activation of this neuron will produce:
(i) an excitatory stimulus to its corresponding third order sensory neuron.
(ii) an inhibitory signal to adjacent neurons A and C via collateral fibers.
The inhibition of neurons A and C diminishes the tonic activity in these neurons making the intensity of B seem greater by contrast.
The discharge of a receptor cell is greatest when a stimulus is applied to the center of a receptive field and weakest at the perimeter.
Receptive fields can be different at the different levels of the sensory pathway i.e. The receptive field of a secondary sensory neuron in the dorsal column can be different from the receptive field of the primary sensory neuron that synapses with it.
Due to activation of inhibitory interneurons by dorsal column secondary sensory neuronsÃ. This inhibition is not present at the level of the receptor
Example à an annular or donnut shaped receptive field (an excitatory center with an inhibitory surround).
3 sensory cells A, B, and C
Activation of primary sensory neuron B (by stimulating within its receptive field) leads to increase in firing level of secondary sensory cell B in dorsal column nucleus.
But:
Activation of cells A and C (by stimulation within their receptive fields) leads to inhibition of cell B because A and C activate inhibitory interneurons that synapse onto cell B.
Result à an annular or donut shaped receptive field for dorsal column secondary sensory cell B
Pain can be modulated by a wide range of behavioral experiences à the joy of childbirth can suppress pain, soldiers wounded in battle and athletes injured in sports events report that they do not feel pain.
Fear of a dentist can intensify pain.
Suggests there are neural mechanisms that can modulate transmission in pain pathways and modify the response to pain.
Nerve fibers that transmit info about pain and temp enter spinal cord through dorsal roots.
Unlike the neurons in the dorsal column pathway the majority of primary sensory neurons in this pathway synapse with secondary sensory neurons located in the spinal cord in the substantia gelatinosa (the uppermost layers of the dorsal horn).
Secondary sensory neurons cross over the midline at the level of the spinal cord and ascend to the brain in the anterolateral pathway which is along the anterior and lateral portion of the spinal cord.
In addition to the thalamus, the secondary sensory neurons make connections with cells in the limbic system and hypothalamus via collateral fibers.
As a result, pain may be accompanied by emotional distress and a variety of autonomic reactions e.g. nausea, vomiting, sweating.
Inputs to the thalamus terminate in the ventrobasal nuclei.
Pain info travels from the thalamus to the primary sensory cortex and the frontal cortex.
Pain info that reaches the primary sensory cortex is associated with acute pain. Acute pain à an adaptive protective response to environmental stress necessary for our survival
Pain info that reaches the frontal cortex is associated with chronic pain à massive problem. In USA more than 2 million people are incapacitated by chronic pain at any given time.
Fast pain à sharp and localized.
Transmitted rapidly by small myelinated fibers (A delta fibers)
Slow Pain à dull, more diffuse Transmitted by small unmyelinated c fibers
Nociceptors exists as free nerve endings that respond to chemical, thermal or mechanical stimuli.
Noxious stimulus activates the nociceptor by depolarizing the membrane of the sensory ending.
When peripheral tissues are injured, the sensation of pain in response to subsequent stimuli is enhanced à hyperalgesia.
Chemical mediators released following tissue injury can sensitize and activate nociceptors
Descending Modulation of Pain Perception
(Fig 8-22)
Pain information can be modified by descending pathways originating in the brain stem and other brain areas
In human patients suffering from chronic pain, stimulating the periaqueductal geay area, the ventrobasal complex of the thalamus or the internal capsule can reduce the severity of the pain.
Mechanism
Primary sensory neurons transmitting pain information release substance P as the neurotransmitter to activate secondary projection neurons in the spinal cord.
Activation of descending inhibitory pathways activate inhibitory interneurons that terminate on the terminals of nociceptive neurons in the spinal cord and inhibit the release of substance P from the sensory fibers.
The inhibitory interneurons release enkephalin which inhibits the release of substance P.
Endogenous Opioid Peptides and their receptors are located at key points in the Pain modulaory system
Sites at which
morphine is effective overlap with those used to evoke
stimulation-induced analgesia.
Indicates that
morphine activates descending pathways that control nociceptive
inputs.
Opiates also
exert a direct analgesic action on the spinal cord.
Analgesia can be
produced by intrathecal injection of opiates into the
subarachnoid space surrounding the spinal cord.
Intrathecal
opiate injection used in certain pain states à e.g labor
pain.
The Gate Theory of Pain Modulation.(See Fig 5 syllabus notes)
Pain can also be suppressed in the dorsal horn before the stimuli are sent to the projection neurons
In the absence of a painful stimulus tonic activity in an inhibitory interneuron prevents ascending signals from going to the brain.
When a painful stimulus activates a nociceptor the c fibre blocks this inhibitory interneuron allowing the c fibre to activate the projection neuron and send the info to the brain.
If a non à nociceptor is activated (e.g. an A beta fiber) it can override the c fibre induced inhibition of the inhibitory interneuron by activating the inhibitory interneuron.
In this way the activation of the secondary projection neuron is reduced and the pain signal diminished.
Application of the Gating Theory à rubbing your elbow or shin when you bump it will activate the A beta fibers which activate the inhibitory interneuron to reduce pain transmission to the brain.
An important part of an organisms response to an emergency is a reduction in pain responsiveness because the set of responses pain usually promotes (reflex withdrawals) could prove to be disadvantageous to the animalÃs survival.
During emergency or stressful situations the reaction to pain can be suppressed in favor of a more adaptive behavior.
Stress activates both opioid and non opioid induced analgesia.
Pain can be felt in the skeletal muscles and viscera as well as in the skin.
Often poorly localized and may be felt in areas far removed form the site of the stimulus à e.g. the pain of cardiac ischemia is often felt in the neck and down the left shooulder and arm.
Mechanism à multiple primary sensory neurons converge onto a single ascending tract in the spinal cord. When a painful stimulus arises from visceral receptors e.g the heart à the brain is unable to distinguish those signals from the more common signals arising from receptors in the skin (somatic regions).
Consequently, it interprets pain as coming from the somatic area instead of the visceral organ.
Amputees often have the sensation of pain emanating from a missing limb.
Chronic overactivity of dorsal horn neurons may convey the illusion that the pain derives from the distal regions of a limb that no longer exists.